To find the answer to this question, you need to refer to lines 16-19 of the poem. Basically, Kirkup is saying that we will go against nature if we take up arms against one another.

What the poet says is that we will "defile" the earth and "outrage" the air that is around us if we fight. But what does he mean by this? To me, what he is saying is that war -- the killing of other human beings -- is an act that is against nature. It is against what we as people are meant to do. So when we do it, we are contaminating the earth and the sky (metaphorically) with the evil of our deeds.

Instead of this, we are all supposed to live together in harmony because that is what nature intends for us (since we are all essentially alike).

The essence of Kirkup's poem is that in attacking others deemed foreign, we end up attacking our own and deny the basic element that makes us human. The challenge here is that there might be some situations where taking up arms against an aggressor is needed. To stop Hitler, for example, attacking "our brothers" was a necessary step. Hitler was not listening to words nor was he paying attention to much else. In rising up against Hitler, it could be argued that more of "our brothers" and sisters were saved from death camps. On a more local level, the situation at Columbine High School might be another moment where "taking up arms" was a necessary step. Dylan Klebold and Eric Harris were approaching students and shooting them based on random questions such as, "Do you believe in God?" Their rampage was borne out of a deep seeded anger that could not have been stopped through discussion and empathy. In the end, arms was needed to stop further aggression and save human life. I think that examples such as these might lie outside the scope of where the poem is intending to take the reader. Another alternative might be that we have to admit that we fall short of Kirkup's standard for universal brotherhood when we take up arms against one another. Yet, it might be a criticism worth taking if such an action is to prevent the loss of others' lives.